Disposable extracorporeal blood circuit and apparatus for the extracorporeal treatment of blood

ABSTRACT

A rigid tubular piece ( 24 ) and a cassette assembly ( 10 ) for extracorporeal blood circulation having the rigid tubular piece are disclosed. The rigid tubular piece is used for submitting fluid circulating therein to measurements and includes a protrusion or a recess in correspondence of at least an end connector. A blood treatment medical apparatus using the cassette assembly is also disclosed. The apparatus is able to detect if the right rigid tubular piece is in proper position before authorizing measurements of a blood property.

FIELD OF THE INVENTION

The invention relates to a disposable extracorporeal blood circuit andto an apparatus for the extracorporeal treatment of blood adapted to usesaid extracorporeal blood circuit.

BACKGROUND OF THE INVENTION

Extracorporeal blood circuits are use to move blood outside the body:blood is typically pumped through tubes and arterial and venous bubbletraps of disposable tubing sets connecting the patient to a bloodtreatment unit, for instance a dialyzer mounted on a dialysis, or to atreatment unit of another type (hemofilter, ultrafilter, hemodiafilter,plasmafilter, etcetera) associated to a corresponding blood treatmentmachine.

Integrated solutions have been used to ease the machine set-up beforetreatment starts.

U.S. Pat. No. 4,263,808 discloses, a one-piece circuit that includesarterial and venous bubble trap chambers in which blood enters atentrances above the bottoms of the chambers and leaves near the bottomsof the chambers. Pressure in the chambers can be determined bytransducers placed against impermeable latex membranes covering holescommunicating with upper portions of the chambers.

U.S. Pat. Nos. 4,798,090 and 4,666,598 disclose a one-piece fluid flowchamber cassette that can be mounted against a supporting machine. Thecassette has a flexible tube that extends from a sidewall and forms aloop that is symmetrical about a loop axis that is transverse to thesidewall so that the loop will be acted upon by a pump roller on themachine when the front wall is against the machine. The flexible tube isconnected at one end to a chamber outlet at the bottom of the chamberand at the other end to the inlet of a flow passage in the cassette,which inlet is located at the same distance from the loop axis as thechamber outlet; the cassette has two chambers (arterial and venouschambers) and additional flexible tubes for connecting a dialyzerbetween the flow passage and the venous chamber; the outlet of thevenous chamber is at the bottom of the venous chamber; and the inlets tothe arterial and venous chambers enter the arterial and venous chambersat locations above the outlets of the chambers. In use with a dialyzer,blood from a patient flows through the arterial chamber, the pump loop,and the flow passage to the dialyzer, and from there through the venouschamber and back to the patient.

It is also known measuring haemoglobin concentration in the bloodcircuit of a dialysis machine: a known way of determining theconcentration of haemoglobin in the red corpuscles during the dialysistreatment, by means of highly accurate measurements of an intrusivekind, which require the laboratory examination of blood samples.

Other dialysis machines enable non-intrusive measurements of thehaemoglobin concentration to be made within the machine. Thenon-intrusive measurements made within the machine are less accuratethan laboratory measurements, but have the advantage of being providedin real time in such a way that the operating parameters of the dialysismachine can be corrected instantaneously. Italian patent IT 1240489discloses a method of measuring the haemoglobin concentration within themachine and in a non-intrusive way, by measuring the absorption ofelectromagnetic waves of the blood flowing in the arterial branch of thefirst circuit.

In order to implement this method a blood circuit, having an arterialline, a venous line and a bubble trap in the arterial line, alsoincluded a calibrated and rigid piece of transparent tube is rigidlyengaged at the outlet of the bubble trap, upstream the connection to thedialyzer. This calibrated piece of transparent tube was designed to bereceived in an appropriate holder where an emitter and sensor operatedto emit and then detect the absorption of electromagnetic waves.

U.S. Pat. No. 6,794,194 discloses another method for measuring thehemoglobin concentration in an extracorporeal blood circuit of adialysis machine comprising the measurement of the values of absorptionby blood of electromagnetic waves conveyed along a section of the saidcircuit; then the calculation of the hemoglobin concentration is made asa function of the values of absorption and the measured value of bloodpressure, blood temperature and the rate of flow of the blood along theaforesaid section.

According to this method a rigid piece of calibrated and transparenttube also including a pressure transducer is interposed between theblood pump and the dialyzer in correspondence of the arterial line, in aposition where an electromagnetic waves sensor and a pressure sensorboth borne by the machine operate.

It is also known to use the measure of haemoglobin concentration as aparameter to control the fluid removal from blood. For instance theultrafiltration rate can be controlled by measuring the bloodhaemoglobin concentration upstream the treatment unit and by keepingsaid haemoglobin concentration or a parameter function of haemoglobinconcentration (such as the filtration factor within a range ofacceptability during treatment.

Finally EP0467805 shows a blood treatment apparatus having anoptical/electronic system comprising an LED diode and a photosensitivesensor capable of receiving the light radiation emitted by the LED andof providing a corresponding electrical signal. A circuit for processingthis electrical signal is able to discriminate when in use a tubethrough which blood flows is placed between LED diode and sensor. Theequipment is provided with a test circuit to check that the parts of theequipment itself are properly operating.

SUMMARY OF THE INVENTION

It is an object of the present invention to render available adisposable cassette assembly bearing a blood pump segment and havingassociated therewith a piece of tubing designed to render themeasurement of a parameter, for instance haemoglobin concentration,reliable and repeatable.

A further object of the invention is a disposable cassette assembly,which is so designed as to isolate the portion of the tube subject tomeasurements from the perturbations coming from actuators acting on theblood pump segment.

An auxiliary object of the invention is a disposable cassette assemblywhere the tube piece designed to be subject to measurements is providedwith appropriate means allowing proper positioning and identification ofthe tube piece so as to avoid that tubes unsuitable to the measurementare used.

According to one embodiment, the disposable cassette assembly is able totrigger generation of a consent signal for the haemoglobin concentrationor related parameter measurement to take place.

According to the present invention the disposable cassette assembly foran extracorporeal fluid treatment medical apparatus of the type havingat least a peristaltic pump, comprises:

-   -   a rigid casing defining at least a first fluid flow chamber,        said casing also having an inlet to said first chamber and an        outlet from said first chamber,    -   a first flexible tube having a first and a second end secured to        the rigid casing to form a portion of a loop designed to receive        a rotor of the medical treatment apparatus peristaltic pump, the        first end of the first flexible tube being connected to said        first chamber outlet,    -   a second flexible tube having a first end connected to the inlet        of said first chamber,    -   a third flexible tube having a first end secured to the rigid        casing and in fluid communication with the second end of said        first tube, wherein the cassette assembly further comprises:        -   a rigid tubular piece having:            -   a first end connector,            -   a second end connector, opposite said first end                connector, and            -   an intermediate portion extending between said first and                second end connectors for subjecting the fluid flowing                through said intermediate portion to a measurement of at                least a fluid parameter, the first end connector of the                rigid tubular piece being secured to one selected in the                group comprising: a second end of the second flexible                tube, and a second end of the third flexible tube.

The cassette assembly can comprises one or more of the followingfeatures:

-   -   said intermediate portion can have a constant cross section.    -   said intermediate portion can have a toric cross section.    -   said intermediate portion can be at least partially transparent        to electromagnetic and/or acoustic waves.    -   when the first end connector of the rigid tubular piece is        secured to the second end of the second flexible tube, and the        second flexible tube can have an axial extension L, an internal        diameter Di, an external diameter De and a hardness H satisfying        at least one of the following conditions:

L/Di>10

2L/(De−Di)>50

H<85 Shore A

to thereby defining a deformable portion between the rigid casing andthe rigid tubular piece.

-   -   when the first end connector of the rigid tubular piece is        secured to the second end of the third flexible tube, and the        third flexible tube may have an axial extension L, an internal        diameter Di, an external diameter De and a hardness H satisfying        at least one of the following conditions:

L/Di>10

2L/(De−Di)>50

H<85 Shore A

to thereby defining a deformable portion between the rigid casing andthe rigid tubular piece.

-   -   said first and second end connectors may have an external        prismatic surface and an internal cylindrical surface, the        external prismatic surface of at least one of said end        connectors bearing a radially protruding element which is        designed to cooperate in use with a corresponding mating recess        provided on a holder of the medical apparatus; the radially        protruding element may be in one piece with the rigid tubular        piece and comprises side walls emerging from external surface of        the end connector and a terminal wall extending transversally to        said side walls; the side walls of the protruding element may be        slightly converging when moving apart from the external surface        of the end connector, and said terminal wall may be        substantially flat and parallel to a longitudinal axis of        development of the rigid tubular piece.    -   said first and second connectors may have an external prismatic        surface and an internal prismatic surface, the external        prismatic surface of at least one of said connectors bearing a        radial recess which is designed to cooperate in use with a        corresponding mating protrusion on a holder of the medical        apparatus. The external prismatic surface may have a pair of        expansions comprising two longitudinal and parallel fins        defining the radial recess.    -   the rigid plastic casing may have front and rear walls spaced        from each other and a side wall between said front and rear        walls to define said first fluid flow chamber.    -   the rigid casing may be a one piece body in transparent plastic        material and wherein the rigid tubular piece is a one piece body        in transparent plastic material.    -   the first end connector of the rigid tubular piece may be        secured to the second end of the second flexible tube, said        second end connector of the rigid tubular piece being secured to        a first end of a fourth flexible tube; in this case the second        end of the fourth flexible tube bears a connector for a vascular        access device and the second end of the third tube bears a        connector to a blood treatment unit.    -   the first end connector of the rigid tubular piece may be        secured to the second end of the third flexible tube, said        second end connector of the rigid tubular piece being secured to        a first end of a fourth flexible tube. In this case the second        end of the second flexible tube bears a connector for a vascular        access device and the second end of the fourth tube bears a        connector to a blood treatment unit.    -   the rigid casing can also comprise an intermediate wall        extending between the side walls to define a second fluid flow        chamber.    -   the rigid casing can also include a rigid channel extending        above the fluid flow chamber and connecting the first end of the        third flexible tube to the second end of said first tube.    -   the second fluid flow chamber may have a second inlet and a        second outlet, said second inlet being secured to an end of a        fifth flexible tube having its other end bearing a connector to        a blood treatment unit and said second outlet being secured to        an end of a sixth flexible tube having its other end bearing a        connector to an access device.

A further object of the invention is a medical apparatus adopting acassette assembly according to the invention.

Another object of the invention is a medical apparatus having theability to cooperate with the rigid tubular piece of the cassetteassembly for carrying our reliable and repeatable detections of theparameter under measure, for instance haemoglobin concentration.

A further object of the invention is a medical apparatus having theability to improve reliability in the detections by insuring properpositioning of the right rigid tubular piece to be subject to measure,in correspondence of the right position.

According to one embodiment, the medical apparatus is able to generate aconsent signal for the haemoglobin concentration measurement to takeplace only when the proper tubular piece in the proper position isdetected.

One or more of the above objects are reached by a cassette assemblyaccording to one of more corresponding aspects of the invention, asdelineated in the appended cassette assembly claims.

One or more of the above objects are also reached by a medical apparatusaccording to one of more corresponding aspects of the invention, asdelineated in the appended medical apparatus claims.

According to the invention a medical apparatus for an extracorporealfluid treatment has:

a main support structure having at least a receiving portion and aholder,a cassette assembly according to anyone of the preceding claims, thecassette assembly being mounted on the receiving portion carried by themain support structure,at least a peristaltic pump carried by the main support structure andhaving a rotor, the portion of loop of the cassette assembly receivingthe rotor of the peristaltic pump,the holder holding the rigid tubular piece of the cassette assembly.

The apparatus may also include one or more of the following features:

-   -   the holder may comprise a base carried by the support structure        and defining a seat for receiving at least the rigid tubular        piece, and a first sensor and comprising at least an emitter of        a signal and a detector which can detect a return signal.    -   the emitter may comprise a waves emitter emitting        electromagnetic or acoustic waves with an emission intensity and        frequency and the detector may comprise a detector of        electromagnetic or acoustic waves which can detect a received        intensity and/or frequency.    -   a control unit connected to the first sensor and including means        for calculating a property of a fluid circulating through the        rigid tubular piece based on one selected in the group        comprising:        -   said emission and received intensities        -   said emission and received frequencies        -   a phase shift between said emitted and received waves.    -   the means for calculating a property of a fluid circulating        through the rigid tubular piece may include:        -   means for calculating a difference or a ratio between the            emission intensity and the received intensity,        -   means for determining an absorption of energy by the fluid            circulating through the rigid tubular piece based on said            difference or on said ratio,        -   means for determining the property of the fluid circulating            through the rigid tubular piece based on said absorption.    -   when the fluid is blood the property of the fluid circulating        through the rigid tubular piece includes at least one selected        in the group comprising: blood density, blood hematocrit, blood        hemoglobin concentration, mean blood cellular volume.    -   the emitter and detector may be positioned in correspondence of        said intermediate portion of the rigid tubular piece.    -   the holder may also comprise a closure element which is coupled        to the base and which can be moved between a closed position,        where it closes the seat and secures in position the rigid        tubular piece, and an open position, where the closure element        leaves the seat open thereby allowing insertion or removal of        the tubular piece into or from the seat. The emitter and        detector may positioned according to one of the configuration        selected in the group comprising: a solution where the closure        element rigidly carries one of said emitter and detector, the        base rigidly carrying the other of said emitter and detector, a        solution where the closure element rigidly carries both said        emitter and detector, a solution where the base rigidly carries        both said emitter and detector.    -   the sensor may have a reflector presenting a reflective surface        entirely surrounding the intermediate portion, the emitter and        detector positioned in correspondence of said reflector.    -   the closure element may include a door hinged around a pivot        axis on one side of the base, the door turning around said pivot        axis when moving between said open and close positions, the door        comprising at least one lock member cooperating with a        counter-lock member on the base, the lock and counter-lock        member being coupled in correspondence of said closed position.        The door may have two lock members, one on each side cooperating        with one respective counter-lock member on the base, the lock        and counter-lock members being coupled in correspondence of said        closed position. Each lock member may comprise an elastically        deformable flat wing having one end secured to the door and one        end carrying a protrusion, in correspondence of said closed        position the protrusion engaging a correspondingly shaped recess        defined on the base.    -   the holder may also comprise a second sensor which is at least        capable of detecting when the closure element is in its closed        position and of emitting a corresponding closure signal.    -   the control unit may be connected to the second sensor and        comprise means for determining whether or not the closure        element is in its closure position based on said closure signal        emitted by the second sensor.    -   the seat may comprise a first portion counter-shaped to first        end connector and a second portion counter-shaped to the second        end connector.    -   the first and second portions of the seat may present a        prismatic surface, the prismatic surface of at least one of said        first and second portions bearing a radial recess which        cooperates with the corresponding mating protrusion present on        one of the end connectors.    -   the first and second portions of the seat may present a        prismatic surface, the prismatic surface of at least one of said        first and second portions bearing a radial protrusion which        cooperates with the corresponding mating recess on one of the        end connectors.    -   a third sensor may be associated to one of said first and second        portions of the seat to detect the presence and proper position        of the rigid tubular piece in the holder seat and emit a        corresponding matching signal.    -   depending upon the case the third sensor may be constructed to        detect presence of a matching coupling between the protrusion on        the end connector and the radial recess on the seat and to emit        a corresponding matching signal or to detect presence of a        matching coupling between protrusion on the seat and radial        recess on the end connector and to emit a corresponding matching        signal.    -   the control unit may be connected to the third sensor and        include means determining whether the proper rigid tubular piece        is in the right position within the seat based on said matching        signal emitted by the third sensor.    -   a treatment unit may be associated to the main support        structure, the treatment unit having a semipermeable membrane        dividing the treatment unit in at least a first chamber for the        flow of blood which is in fluid communication with the cassette        assembly and in a second chamber which has at least an outlet        port in fluid communication with an outlet line, and there may        be provision for means for regulating the net flow of plasma        water passing through the semipermeable membrane from the first        to the second chamber and viceversa. Means may be present for        controlling the regulating means based on values taken by said        calculated property.    -   The control unit may include means for determining whether the        proper rigid tubular piece is in the right position within the        seat based on said matching signal emitted by the third sensor.        The control unit comprises means for commanding the means for        activating the first sensor only when the means for determining        have positively concluded that the proper rigid tubular piece is        in the right position within the seat.    -   The control unit may also comprise means for signaling that the        fluid property cannot be calculated when the means for        determining have not positively concluded that the proper rigid        tubular piece is in the right position within the seat.    -   The control unit may also comprise means for activating said        means for controlling the regulating means only when the means        for determining have positively concluded that the proper rigid        tubular piece is in the right position within the seat.    -   When the apparatus comprises a supply line for a treatment        liquid connected at an inlet of the second chamber of the blood        treatment unit, then means for regulating a chemical composition        of the treatment liquid in the supply line may be present, and        the control unit may also comprise means controlling the means        for regulating the chemical composition based on values taken by        said calculated property.

Finally the invention concerns a rigid tubular piece for andextracorporeal blood circuit, the rigid tubular piece having:

-   -   a first end connector,    -   a second end connector, opposite said first end connector, and    -   an intermediate portion extending between said first and second        end connectors for subjecting the fluid flowing through said        intermediate portion to a measurement of at least a fluid        parameter, le tube.

Further possible features of the rigid tubular piece and of a cassetteassembly rigidly bearing the rigid tubular piece are described in claims56 to 66.

SHORT DESCRIPTION OF THE DRAWINGS

Further features and advantages will be best understood from thedetailed description of some non limiting embodiments of the presentinvention. This description will be carried out hereinafter withreference to the accompanying drawings, also given by way ofnon-limiting example, in which:

FIG. 1 is an elevation of a first embodiment of a cassette assembly foruse with a fluid treatment medical apparatus.

FIG. 1A is a side view of a portion of the embodiment of FIG. 1.

FIG. 2 is an elevation of a second embodiment of cassette assembly foruse with a fluid treatment medical apparatus.

FIG. 2A is a side view of a portion of the embodiment of FIG. 2.

FIG. 3 is an enlarged perspective view of a rigid tubular piece for usein the cassette assembly of FIG. 1 or FIG. 2.

FIG. 4 is a front view of the rigid tubular piece of FIG. 3.

FIG. 5 is a side view of the rigid tubular piece of FIG. 3.

FIG. 6 is a section view along section plane VI-VI of FIG. 5.

FIG. 7 is a plan view of the rigid tubular piece of FIG. 5.

FIG. 8 is a section view along section plane VIII-VIII of FIG. 4.

FIG. 9 is a section view along section plane IX-IX of FIG. 5.

FIG. 10 is an enlarged view of a portion of the section view of FIG. 6.

FIG. 11 is an interrupted perspective view of a medical apparatus whichcan receive the cassette assembly of FIG. 1.

FIG. 12 is a perspective view concerning an holder of the apparatus ofFIG. 11, in closed position.

FIG. 13 is a perspective view concerning an holder of the apparatus ofFIG. 11, in open position.

FIG. 14 shows an enlarged plan view of an alternative embodiment of therigid tubular piece associated with the cassette assembly of FIGS. 1 and2.

FIG. 15 shows an enlarged and interrupted side view of the rigid tubularpiece of FIG. 14.

FIGS. 16 and 17 show a cross and longitudinal section of the seat wherethe rigid tubular piece of FIG. 14 can operate.

FIGS. 18 and 19 show in interrupted cross sectional view an alternativeshape for the seat and the rigid tubular piece.

FIG. 20 is a schematic diagram of a blood treatment machine.

DETAILED DESCRIPTION

With reference to the enclosed drawings, reference number 1 denotes amedical apparatus for an extracorporeal fluid treatment. In particularthe apparatus can be an apparatus for the treatment of blood, such as byway of non-limiting example a machine for the treatment of renal orliver insufficiency. In the example shown in the attached figures, themedical apparatus 1 is a machine for one or more of the followingextracorporeal blood treatments: hemodialysis, hemofiltration,ultrafiltration, hemodiafiltration, and plasma-aphaeresis.

The apparatus 1 comprises a main support structure 2 and operating panel3, which can be in a front position of the apparatus, including a userinterface 4 (only schematically represented), one or more pumps 5(volumetric pumps of the type acting in deformation of deformable tubeportions, such as peristaltic pumps), and at least a holder 6 soconstructed as to receive a rigid tubular piece to be subject to a noninvasive measurement as it will be explained in detail herein below.

The apparatus also presents an auxiliary holder 7 for receiving at leasta blood treatment unit 8 (for instance a dialyzer or an ultrafilter oran hemofilter or an hemodiafilter or a plasmafilter). The bloodtreatment unit can comprise, in a manner per se known and therefore notfurther detailed, a first and a second compartment separated by asemipermeable membrane. The first compartment is for the passage ofblood and the second compartment is for the passage of discardedsubstances and/or treatment liquid.

The medical apparatus has also a receiving portion 9 for holding acassette assembly 10. In the enclosed figures the receiving portion ison the front panel 3 in close proximity of the peristaltic pump 5 andcooperates with a pivoting door which serves to secure in position thecassette assembly and the portion of tubing cooperating with the rotor.

The disposable cassette assembly 10 comprises a rigid casing 11 definingat least a first fluid flow chamber 12; in reality in the embodimentshown the rigid casing defines two chambers 12, 13. For instance, inorder to define a two chambers rigid structure, the rigid casing canhave front and rear walls spaced from each other and side walls betweensaid front and rear walls as well as an intermediate wall whichcooperates with the side walls to define said two chambers. Pressuretransducers 12 a and 13 a can be associated to each one of the twochambers walls, in a manner per se known and not further detailed.

When there is the need to see the material flowing inside the cassetteassembly, as it is desirable in case of blood treatments, then rigidcasing can be in transparent plastic material.

The rigid casing, for instance made in one piece transparent PVCmaterial, also defines an inlet 14 to the first chamber 12 and an outlet15 from said first chamber. In the embodiment of FIG. 1 both the inlet14 and the outlet 15 are placed in correspondence of an inferior portionof the rigid casing 11.

The cassette assembly comprises a plurality of flexible tubes engaged tothe rigid casing. More in detail, a first flexible tube 16 has a firstand a second end 17 and 18 secured to the rigid casing to form a portionof a loop 19 designed to receive the rotor of peristaltic pump 5: thefirst end 17 of the first flexible tube is connected to said firstchamber outlet 15 while the second end 18 of the first flexible tube issecured to the casing in a position which is vertically displaced withrespect to the first end, as shown in FIGS. 1 and 2.

A second flexible tube 20 has a first end 21 connected to the inlet 14of said first chamber 12. A third flexible tube 22 has a first end 23secured to the rigid casing 11 and in fluid communication with thesecond end 18 of said first tube 16.

The cassette assembly 10 further comprises a rigid tubular piece 24having a first end connector 25, a second end connector 26 opposite saidfirst end connector 25, and an intermediate portion 27 extending betweensaid first and second end connectors 25, 26. The intermediate portion 27is designed and calibrated because in use, when it is flown through byblood or other fluid, it is adopted for subjecting the fluid flowingthrough said intermediate portion 27 to non-invasive measurement ofcertain parameters of the fluid. The intermediate portion 27 may have aconstant cross section and may be made of at least partially transparentmaterial.

Of course the cross section could also be variable but this may entail amore complex measurement procedure. In term of shape, again referring tothe embodiment shown, the intermediate portion 27 can have a toric crosssection.

In the embodiment shown the rigid tubular piece 24 is in a single piecemade in transparent plastic material, for instance PVC (of course otherrigid plastic materials can alternatively be used).

in the embodiment of FIG. 2 the first end connector 25 of the rigidtubular piece 24 is secured to the second end 28 of the second flexibletube 20. In this case, the second flexible tube 20 has a prefixed axialextension sufficient to absorb, by axial deformation or by bending, allrelative displacements between the rigid casing 11 and the rigid tubularpiece 24 when, in use, they are in mounted on the medical apparatus 1.

For instance, the second flexible tube 20 can be equal to or longer thanthe axial length of the rigid tubular piece 24. According to anotheraspect, the axial length of the second flexible tube 20 of FIG. 2 can begreater than three times its external diameter.

In accordance with a further, alternative or additional, aspect thesecond flexible tube has an axial extension L, an internal diameter Di,an external diameter De and a hardness H satisfying at least one of thefollowing conditions:

L/Di>10

2L/(De−Di)>50

H<85 Shore A

to thereby defining a deformable portion between the rigid casing andthe rigid tubular piece thereby isolating the holder from vibrationswhich may deteriorate quality of the measurements of said property.

In the example shown:

L/Di>18

2L/(De−Di)>55

H<80 Shore A

In the case of FIG. 2 the second end connector 26 of the rigid tubularpiece 24 is secured to a first end 29 of a fourth flexible tube 30,while the second end 31 of the fourth flexible tube 30 bears a connector32 for a vascular access device (such as a cannula, a needle, acatheter, or other access) and the second end 33 of the third tube 22bears a connector 34 for coupling to a corresponding counter connectoron the inlet port of the blood treatment unit first chamber (not shown).

In the embodiment of FIG. 1 the first end connector 25 of the rigidtubular piece 24 is secured to the second end 33 of the third flexibletube 22. In this case, the third flexible tube 22 has a prefixed axialextension which is sufficient to absorb, by axial deformation or bybending, all relative displacements between the rigid casing 11 and therigid tubular piece 24 when, in use, they are in mounted on the medicalapparatus 1.

For instance, the third flexible tube 22 can be equal to or longer thanthe axial length of the rigid tubular piece 24.

According to another aspect, the third flexible tube has an axialextension L, an internal diameter Di, an external diameter De and ahardness H satisfying at least one of the following conditions:

L/Di>10

2L/(De−Di)>50

H<85 Shore A

to thereby defining a deformable portion between the rigid casing andthe rigid tubular piece which is able to absorb any relative movementbetween the rigid casing and the holder, thereby isolating the holderfrom vibrations which may deteriorate quality of the measurements ofsaid property.

In the example shown:

L/Di>18

2 L/(De−Di)>55

H<80 Shore A

In the case of FIG. 1 the second end connector 26 of the rigid tubularpiece 24 is secured to a first end 29 of a fourth flexible tube 30 andthe second end 31 of the fourth tube 30 bears a connector 34 tocorresponding counter connector on the inlet port of the blood treatmentunit first chamber (not shown). In the example of FIG. 1 the secondflexible tube 20 bears at a second end thereof a connector 32 for avascular access device (such as a cannula, a needle, a catheter, orother access).

In both the embodiments of FIGS. 1 and 2 the cassette assembly 10includes a rigid channel 35 extending above the fluid flow chambers 12,13 and connecting the first end 23 of the third flexible tube 22 to thesecond end 18 of said first tube 16. Moreover the second fluid flowchamber 13 has a second inlet 36 and a second outlet 37 (where a venousfilter 38 can operate): the second inlet 36 is secured to an end of afifth flexible tube 39 having its other end bearing a connector 40 to ablood treatment unit for coupling to a corresponding counter connectoron the outlet port of the blood treatment unit first chamber (notshown). The second outlet 37 of the second chamber 13 is secured to anend of a sixth flexible tube 41 having its other end bearing a connector42 to an access device (such as a cannula, a needle, a catheter, orother access).

In the embodiments shown mounting means 43 is associated to the cassetteassembly for mounting said casing on said machine. In this case themounting means comprises at least one projection 44 rigidly carried bythe rigid casing. The mounting means 43 also include tubular extensions45, 46 rigidly carried on one side of the rigid casing 11 and secured toa respective opposite end of said first flexible tube 16. This mountingmeans 43 cooperates with corresponding pick-up elements on the frontpanel of the machine in correspondence of said receiving portion. Ofcourse other mounting means could be provided for, such as extensions orrecesses in positions different from those above described, or anappropriate shaping of the cassette assembly walls and correspondingreceiving portion and so on.

Going now into a more detailed description of the rigid tubular piece 24(see FIGS. 3 to 9, 14, 15), the first and second end connectors 25, 26have an external prismatic, for instance cylindrical, surface and aninternal prismatic surface; the internal prismatic surface of each endconnector in the embodiments shown presents a main cylindrical tract 25a, 26 a and a frustum-conical leading edge 25 b, 26 b. The cylindricaltracts have a diameter greater than that of the intermediate portioninner surface 27 a in order to define an abutment 25 c, 26 c for theconnection of the tube end portions. Notice that the intermediateportion 27 internal surface 27 a can be prismatic, and in this case iscylindrical, and extends in immediate prosecution of the abutment. Inpractice in order to avoid stagnation areas, the diameter of thecylindrical tracts 25 a,26 a can be made equal to that of theintermediate portion inner surface 27 a plus two times the thickness ofthe end portions of the tube secured in correspondence of said endconnectors. This assures that in use a continuous and smooth channel iscreated through the whole rigid tubular piece.

The external prismatic surface of at least one of said end connectorsbears a radially protruding element 47 which is designed to cooperate inuse with a corresponding mating recess 48 provided on the holder 6 ofthe medical apparatus.

The radially protruding element 47 is in one piece with the rigidtubular piece 24 and comprises side walls 49 emerging from externalsurface of one of the end connectors and a terminal wall 50 extendingtransversally to side walls 49. In the embodiment shown two oppositeside walls 49 can protrude from the surface of the end connector andterminate into the terminal wall 50. The protrusion can also present twoopposite side indents 51, which in the example of FIGS. 3-10 extendinside the protrusion conferring to the protrusion a cross section inthe shape of a T (see FIG. 8).

The opposite side walls of the protruding element can be slightlyconverging (see FIGS. 6 and 10), at least in correspondence of aterminal portion thereof, when moving apart from the external surface ofthe end connector; the terminal wall is substantially flat and parallelto a longitudinal axis of development of the rigid tubular piece.

Alternatively or in combination with the protrusion (please refer to theexample of FIGS. 14-17, one of the end connectors 25, 26 can present oneor more recesses 52 on an external surface thereof. In particular,referring to the embodiment shown, the first and second connectors whichhave an external prismatic surface and an internal cylindrical surfacecan bear a pair of expansions 53 defining a radial recess 52 which isdesigned to cooperate in use with a corresponding mating protrusion 54on the holder of the medical apparatus. In the embodiment shown, theexpansions comprise two longitudinal and parallel fins providing aradial recess with a substantially U-shaped cross section. When theprotrusion mates with the recess 52, the expansions 53 engagecounter-shaped cavities 55 on the holder 6

Going now back to the overall apparatus, apparatus 1 of the enclosedfigures comprises, as mentioned, the holder 6 for the rigid tubularpiece 24. This holder may include a base 56 carried by the supportstructure and defining a seat 57 for receiving at least the rigidtubular piece 24, and a first sensor 58 associated to the base andcomprising at least an emitter 59 of a signal and a detector 60 whichcan detect a return signal. The rigid tubular piece comprises at least aportion which is transparent or at least partially transparent to saidsignals in order to allow a non invasive measurement made taking intoaccount the influence of the fluid on said signals.

In greater detail, the emitter comprises a waves emitter emittingelectromagnetic or acoustic waves with specified emission property (e.g.specified intensity or frequency) and the detector comprises a detectorof electromagnetic or acoustic waves which can detect a receivedintensity or frequency or phase.

A control unit 61 connected to the first sensor includes means forcalculating a property of a fluid circulating through the rigid tubularpiece based on said emission and received intensities or on the phaseshift between the emitted and received signals or on alteration of thefrequency between emitted and received signal.

In the embodiment now described, the means for calculating a property ofa fluid circulating through the rigid tubular piece can include:

means for calculating a difference or a ratio between the emissionintensity and the received intensity,means for determining an absorption of energy by the fluid circulatingthrough the rigid tubular piece based on said difference or on saidratio, means for determining the property of the fluid circulatingthrough the rigid tubular piece based on said absorption.

When the fluid is blood, the property of the fluid circulating throughthe rigid tubular piece includes at least one selected in the groupcomprising:

-   -   blood density,    -   blood hematocrit,    -   blood hemoglobin concentration,    -   mean blood cellular volume.

The emitter and detector 59 and 60 are positioned in correspondence ofsaid intermediate portion, which is at least in part transparent to saidacoustic or electromagnetic waves, of the rigid tubular piece.

The holder also comprises a closure element 62 which is coupled to thebase and which can be moved between a closed position, where it closesthe seat and secures in position the rigid tubular piece, and an openposition, where the closure element leaves the seat open therebyallowing insertion or removal of the tubular piece into or from theseat. The closure element 62 rigidly can carry one of said emitter anddetector, while the base rigidly carries the other of said emitter anddetector. Alternatively both emitter and detector can be on the base orboth on the closure element. When the closure element is closed onto thebase, the emitter and detector act in direct contact to the intermediateportion external surface. In the embodiment shown, the closure elementincludes a door hinged around a pivot axis 63 on one side of the base,so that the door can turn around said pivot axis when moving between theopen and close positions.

The door comprises at least one lock member 64 cooperating with acounter-lock member 65 on the base 56. When the door is in the closedposition, the lock and counter-lock member are coupled the one with theother as shown in FIG. 12, leaving two openings 66 for the passage ofthe tube ends connected with the rigid tubular piece 24.

In the example, two lock members 64, one on each side are cooperatingwith respective counter-lock members on each side of the base and extendin an area 67 of the holder opposite to the pivot axis so that whenclosing the door the rigid tube portion and its seat in the holderextends between the pivot axis and the door lock and counter-lockmembers.

Each lock member can comprise an elastically deformable flat wing havingone end 64 a secured to the door and one end 64 b carrying a protrusion68 designed for engaging a correspondingly shaped counter-lock member 65in the form of a recess (in the embodiment a through slit) defined onthe base.

The holder 6 also comprises a second sensor 69 which is at least capableof detecting when the closure element is in its closed position and ofemitting a corresponding closure signal. Of course the sensor 69 couldbe more sophisticated and be able to monitor each position of theclosure element. In the embodiment shown the sensor is a hall sensor 69a housed on the base 56 and cooperating with a magnet 69 b on the door.Of course one could use alternative solutions, such as a switch or arelays or other equivalent solutions able to detect closure of the door62. The control unit is also connected to the second sensor andcomprises means for determining whether or not the closure element is inits closure position based on said closure signal emitted by the secondsensor.

The holder can also present a third sensor 70 operatively associated toone of said first and second portions of the seat to detect the presenceand proper position of the rigid tubular piece in the holder seat andemit a corresponding matching signal. More in detail the third sensorcan be associated to one of said first and second portions incorrespondence or in close proximity of said radial recess 48 (or radialprotrusion 54 as the case may be) of the seat. The third sensor isconstructed to detect presence of a matching coupling between theprotrusion 47 on the end connector and the radial recess on the seat andto emit a corresponding matching signal.

Alternatively if the embodiment of FIGS. 14-17 the third sensor isoperatively associated to one of said first and second portions incorrespondence of the radial protrusion of the seat and is constructedto detect presence of a matching coupling between protrusion 54 on theseat and radial recess 52 on the end connector and to emit acorresponding matching signal. In reality in the embodiment of FIG. 16the sensor 70 is placed on the protrusion and detects presence of thefins 53 in the cavities 55. Channels 70 a may be provided to send anoptical information when fins 53 occupy the cavities 55.

The further alternative of FIGS. 18 and 19 shows an alternative shapefor the protrusion on the seat of holder 6 and the recess 52 on the endconnector or connectors.

In any case the control unit connected to the third sensor includesmeans for determining whether the proper rigid tubular piece is in theright position within the seat based on said matching signal emitted bythe third sensor.

As mentioned a treatment unit 8 is associated to the main supportstructure, the treatment unit having a semipermeable membrane 8 adividing the treatment unit in at least a first chamber 71 for the flowof blood which is in fluid communication with the cassette assembly 9and in a second chamber 72 which has at least an outlet port in fluidcommunication with an outlet line 73

The apparatus may include means for regulating 74 the net flow of plasmawater passing through the semipermeable membrane from the first to thesecond chamber and viceversa. The means 74 are only schematically shownin FIG. 20 as their structure is not relevant (they could include anultrafiltration pump or two pumps one on the fresh treatment liquidinlet line 75 and one on the outlet line 73 which when driven atdifferent speed determine ultrafiltration or backfiltration into theblood circuit 76, or even blood pumps in the blood circuit which can bedriven at different speed).

Moreover the apparatus may have means 76 for regulating a chemicalcomposition of the treatment liquid in the supply line (also in thiscase details are not given because the specific structure is notrelevant; by way of example the means 77 may comprise one or moreconcentrates in liquid and or powder state connected to the line 75 andrespective flow regulators such as valves or pumps which are controlledby the control unit to reach de desired composition of the treatmentliquid).

The control unit 61 also comprises means controlling the means 77 forregulating the chemical composition based on values taken by saidcalculated property. More in detail the control unit may have one ormore of the following means:

-   -   a) means for determining whether the proper rigid tubular piece        is in the right position within the seat based on said matching        signal emitted by the third sensor,    -   b) means for activating said first sensor to emit said signal        and detect said return signal,    -   c) means for calculating a property of a fluid circulating        through the rigid tubular piece based on said emission and        return signals,    -   d) means for controlling the regulating means 74 based on values        taken by said calculated property,    -   e) means for controlling the means for regulating the chemical        composition 77 based on values taken by said calculated        property.

The control unit comprises timing means for sequentially activating theabove means a) to e).

The control unit may also comprise the further following means.

According to one aspect of the invention the control unit has means forcommanding the activating means only when the means for determining havepositively concluded that the proper rigid tubular piece is in the rightposition within the seat.

According to a further aspect of the invention the control unit also hasmeans for signaling that the fluid property cannot be calculated whenthe means for determining have not positively concluded that the properrigid tubular piece is in the right position within the seat.

According to another aspect of the invention the control unit has meansfor activating said means for controlling the regulating means only whenthe means for determining have positively concluded that the properrigid tubular piece is in the right position within the seat.

According to another aspect of the invention the control unit has meansfor activating said means for regulating a chemical composition of thetreatment liquid in the supply line only when the means for determininghave positively concluded that the proper rigid tubular piece is in theright position within the seat.

Depending upon the case, the control unit can be an entirely digitalunit comprising one or more CPUs, or an analogical control unit, or acombination of the two types just mentioned.

When the control unit is of the analogical type, each of the abovelisted means of the control unit is defined by a correspondinganalogical circuitry in the control unit suitably designed for carryingout the specified function.

When, on the other hand, the control unit is in part or totally of thedigital type, then each of the above listed means of the control unit isdefined by the CPU when running an appropriate software program orsoftware routine which makes the CPU programmed to define one or more ofthe above listed control unit means.

1-66. (canceled)
 67. A disposable cassette assembly for an extracorporeal fluid treatment medical apparatus of the type having at least a peristaltic pump, the cassette assembly comprising: a rigid casing defining at least a first fluid flow chamber, said casing also having an inlet to said first chamber and an outlet from said first chamber, a first flexible tube having a first and a second end secured to the rigid casing to form a portion of a loop designed to receive a rotor of the medical treatment apparatus peristaltic pump, the first end of the first flexible tube being connected to said first chamber outlet, a second flexible tube having a first end connected to the inlet of said first chamber, a third flexible tube having a first end secured to the rigid casing and in fluid communication with the second end of said first tube, characterized in that the cassette assembly further comprises: a rigid tubular piece having: a first end connector, a second end connector, opposite said first end connector, and an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, the first end connector of the rigid tubular piece being secured to one selected in the group comprising: a second end of the second flexible tube, and a second end of the third flexible tube; said first and second end connectors having an external prismatic surface, the external prismatic surface of at least one of said end connectors bearing a radially protruding element which is designed to cooperate in use with a corresponding mating recess provided on a holder of the medical apparatus, the radially protruding element comprising side walls emerging from external surface of the end connector and a terminal wall extending transversally to said side walls.
 68. Cassette assembly according to claim 67, wherein said intermediate portion has a constant cross section and is at least partially transparent to electromagnetic and/or acoustic waves, said intermediate portion having a toric cross section.
 69. Cassette assembly according to claim 67, wherein the first end connector of the rigid tubular piece is secured to the second end of the second flexible tube, and the second flexible tube has an axial extension L, an internal diameter Di, an external diameter De and a hardness H satisfying at least one of the following conditions: L/Di>10 2L/(De−Di)>50 H<85 Shore A to thereby defining a deformable portion between the rigid casing and the rigid tubular piece.
 70. Cassette assembly according to claim 67, wherein the first end connector of the rigid tubular piece is secured to the second end of the third flexible tube, and the third flexible tube has an axial extension L, an internal diameter Di, an external diameter De and a hardness H satisfying at least one of the following conditions: L/Di>10 2L/(De−Di)>50 H<85 Shore A to thereby defining a deformable portion between the rigid casing and the rigid tubular piece.
 71. Cassette assembly according to claim 67, wherein the side walls of the protruding element are slightly converging when moving apart from the external surface of the end connector, and said terminal wall is substantially flat and parallel to a longitudinal axis of development of the rigid tubular piece.
 72. Cassette assembly according to according to claim 67, wherein the rigid plastic casing has front and rear walls spaced from each other and a side wall between said front and rear walls to define said first fluid flow chamber, said rigid casing also comprises an intermediate wall extending between the side walls to define a second fluid flow chamber, the rigid casing also including a rigid channel extending above the fluid flow chamber and connecting the first end of the third flexible tube to the second end of said first tube.
 73. Cassette assembly according to claim 67, wherein either the first end connector of the rigid tubular piece is secured to the second end of the second flexible tube, said second end connector of the rigid tubular piece being secured to a first end of a fourth flexible tube and wherein the second end of the fourth flexible tube bears a connector for a vascular access device and the second end of the third tube bears a connector to a blood treatment unit, or the first end connector of the rigid tubular piece is secured to the second end of the third flexible tube, said second end connector of the rigid tubular piece being secured to a first end of a fourth flexible tube and wherein the second end of the second flexible tube bears a connector for a vascular access device and the second end of the fourth tube bears a connector to a blood treatment unit, the second fluid flow chamber having a second inlet and a second outlet, said second inlet being secured to an end of a fifth flexible tube having its other end bearing a connector to a blood treatment unit and said second outlet being secured to an end of a sixth flexible tube having its other end bearing a connector to an access device.
 74. Cassette assembly according to claim 67 comprising mounting means for mounting said casing on said machine, the mounting means comprising at least one projection rigidly carried by the rigid casing and also include tubular extensions rigidly carried on one side of the rigid casing and secured to a respective opposite end of said first flexible tube.
 75. A disposable cassette assembly for an extracorporeal fluid treatment medical apparatus of the type having at least a peristaltic pump, the cassette assembly comprising: a rigid casing defining at least a first fluid flow chamber, said casing also having an inlet to said first chamber and an outlet from said first chamber, a first flexible tube having a first and a second end secured to the rigid casing to form a portion of a loop designed to receive a rotor of the medical treatment apparatus peristaltic pump, the first end of the first flexible tube being connected to said first chamber outlet, a second flexible tube having a first end connected to the inlet of said first chamber, a third flexible tube having a first end secured to the rigid casing and in fluid communication with the second end of said first tube, characterized in that the cassette assembly further comprises: a rigid tubular piece having: a first end connector, a second end connector, opposite said first end connector, and an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, said intermediate portion having an external prismatic surface and being at least partially transparent to electromagnetic and/or acoustic waves, said intermediate portion having a toric cross section, the first end connector of the rigid tubular piece being secured to one selected in the group comprising: a second end of the second flexible tube, and a second end of the third flexible tube; said intermediate portion having an external prismatic surface and. at least partially transparent to electromagnetic and/or acoustic waves, said intermediate portion having a toric cross section.
 76. Cassette assembly according to claim 75, wherein said first and second connectors have an external prismatic surface and an internal prismatic surface, the external prismatic surface of at least one of said connectors bearing a radial recess which is designed to cooperate in use with a corresponding mating protrusion on a holder of the medical apparatus.
 77. Cassette assembly according to claim 76, wherein the external prismatic surface has a pair of expansions comprising two longitudinal and parallel fins defining the radial recess.
 78. A medical apparatus for an extracorporeal fluid treatment having: a main support structure having at least a receiving portion and a holder, a cassette assembly comprising: a rigid casing defining at least a first fluid flow chamber, said casing also having an inlet to said first chamber and an outlet from said first chamber, a first flexible tube having a first and a second end secured to the rigid casing to form a portion of a loop designed to receive a rotor of the medical treatment apparatus peristaltic pump, the first end of the first flexible tube being connected to said first chamber outlet, a second flexible tube having a first end connected to the inlet of said first chamber, a third flexible tube having a first end secured to the rigid casing and in fluid communication with the second end of said first tube, characterized in that the cassette assembly further comprises: a rigid tubular piece having: a first end connector, a second end connector, opposite said first end connector, and an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, the first end connector of the rigid tubular piece being secured to one selected in the group comprising: a second end of the second flexible tube, and a second end of the third flexible tube, the cassette assembly being mounted on the receiving portion carried by the main support structure, at least a peristaltic pump carried by the main support structure and having a rotor, the portion of loop of the cassette assembly receiving the rotor of the peristaltic pump, the holder holding the rigid tubular piece of the cassette assembly, said holder comprising: a base carried by the support structure and defining a seat for receiving at least the rigid tubular piece, and a first sensor and comprising at least an emitter of a signal and a detector which can detect a return signal.
 79. Apparatus according to claim 78, wherein the emitter comprises a waves emitter emitting electromagnetic or acoustic waves with an emission intensity and frequency and the detector comprises a detector of electromagnetic or acoustic waves which can detect a received intensity and/or frequency, the apparatus comprising a control unit connected to the first sensor and including means for calculating a property of a fluid circulating through the rigid tubular piece based on one selected in the group comprising: said emission and received intensities said emission and received frequencies a phase shift between said emitted and received waves.
 80. Apparatus according to claim 79, wherein the means for calculating a property of a fluid circulating through the rigid tubular piece includes: means for calculating a difference or a ratio between the emission intensity and the received intensity, means for determining an absorption of energy by the fluid circulating through the rigid tubular piece based on said difference or on said ratio, means for determining the property of the fluid circulating through the rigid tubular piece based on said absorption and wherein the fluid is blood and the property of the fluid circulating through the rigid tubular piece includes at least one selected in the group comprising: blood density, blood hematocrit, blood hemoglobin concentration, mean blood cellular volume.
 81. Apparatus according to claim 78, wherein the said emitter and detector are positioned in correspondence of said intermediate portion of the rigid tubular piece and wherein the emitter and detector are positioned according to one of the configuration selected in the group comprising: a solution where the closure element rigidly carries one of said emitter and detector, the base rigidly carrying the other of said emitter and detector, a solution where the closure element rigidly carries both said emitter and detector, a solution where the base rigidly carries both said emitter and detector, the sensor comprising a reflector presenting a reflective surface entirely surrounding the intermediate portion, the emitter and detector positioned in correspondence of said reflector.
 82. Apparatus according to claim 79, wherein the holder also comprises a closure element which is coupled to the base and which can be moved between a closed position, where it closes the seat and secures in position the rigid tubular piece, and an open position, where the closure element leaves the seat open thereby allowing insertion or removal of the tubular piece into or from the seat, the closure element including a door hinged around a pivot axis on one side of the base, the door turning around said pivot axis when moving between said open and close positions, the door comprising at least one lock member cooperating with a counter-lock member on the base, the lock and counter-lock member being coupled in correspondence of said closed position, the door comprises two lock members, one on each side cooperating with one respective counter-lock member on the base, the lock and counter-lock members being coupled in correspondence of said closed position, possibly each lock member comprising an elastically deformable flat wing having one end secured to the door and one end carrying a protrusion, in correspondence of said closed position the protrusion engaging a correspondingly shaped recess defined on the base.
 83. Apparatus according to claim 82 wherein the holder also comprises a second sensor which is at least capable of detecting when the closure element is in its closed position and of emitting a corresponding closure signal, the control unit being connected to the second sensor and comprises means for determining whether or not the closure element is in its closure position based on said closure signal emitted by the second sensor.
 84. Apparatus according to claim 78, wherein the seat comprises a first portion counter-shaped to first end connector and a second portion counter-shaped to the second end connector, said first and second portions of the seat presenting a prismatic surface, the prismatic surface of at least one of said first and second portions bearing a radial recess which cooperates with the corresponding mating protrusion present on one of the end connectors.
 85. Apparatus according to claim 84, wherein said first and second portions of the seat present a prismatic surface, the prismatic surface of at least one of said first and second portions bearing a radial recess which cooperates with the corresponding mating protrusion present on one of the end connectors.
 86. Apparatus according to claim 84, wherein said first and second portions of the seat present a prismatic surface, the prismatic surface of at least one of said first and second portions bearing a radial protrusion which cooperates with the corresponding mating recess on one of the end connectors.
 87. Apparatus according to claim 78 including a third sensor operatively associated to one of said first and second portions of the seat to detect the presence and proper position of the rigid tubular piece in the holder seat and emit a corresponding matching signal, and including a control unit connected to the third sensor and having means determining whether the proper rigid tubular piece is in the right position within the seat based on said matching signal emitted by the third sensor, the third sensor being constructed to detect presence of a matching coupling between the protrusion on the end connector and the radial recess on the seat and to emit a corresponding matching signal, in detail the control unit comprising means for commanding the activating means only when the means for determining have positively concluded that the proper rigid tubular piece is in the right position within the seat, the control unit also comprising means for signaling that the fluid property cannot be calculated when the means for determining have not positively concluded that the proper rigid tubular piece is in the right position within the seat.
 88. Apparatus according to claim 78, comprising: a treatment unit associated to the main support structure, the treatment unit having a semipermeable membrane dividing the treatment unit in at least a first chamber for the flow of blood which is in fluid communication with the cassette assembly and in a second chamber which has at least an outlet port in fluid communication with an outlet line, means for regulating the net flow of plasma water passing through the semipermeable membrane from the first to the second chamber and viceversa and wherein the apparatus comprises: a supply line for a treatment liquid, the supply line being connected at an inlet of the second chamber of the blood treatment unit, means for regulating a chemical composition of the treatment liquid in the supply line, the control unit also comprising means controlling the means for regulating the chemical composition based on values taken by said calculated property.
 89. Apparatus according to claim 88, wherein the emitter comprises a waves emitter emitting electromagnetic or acoustic waves with specified emission intensity and the detector comprises a detector of electromagnetic or acoustic waves which can detect a received intensity, the apparatus further comprising a control unit connected to the first sensor and to the regulating means, the control unit including: means for activating said first sensor to emit said signal and detect said return signal, means for calculating a property of a fluid circulating through the rigid tubular piece based on said emission and return signals, and means for controlling the regulating means based on values taken by said calculated property, the means for calculating a property of a fluid circulating through the rigid tubular piece includes: means for calculating a difference or a ratio between the emission intensity and the received intensity, means for determining an absorption of energy by the fluid circulating through the rigid tubular piece based on said difference or on said ratio, means for determining the property of the fluid circulating through the rigid tubular piece based on said absorption, the fluid being blood and the property of the fluid circulating through the rigid tubular piece including at least one selected in the group comprising: blood density, blood hematocrit, blood hemoglobin concentration, mean blood cellular volume.
 90. A rigid tubular piece for and extracorporeal blood circuit, the rigid tubular piece having: a first end connector having an external prismatic surface, a second end connector, opposite said first end connector, and having an external prismatic surface; an intermediate portion extending between said first and second end connectors for subjecting the fluid flowing through said intermediate portion to a measurement of at least a fluid parameter, characterized in that the external prismatic surface of at least one of said end connectors bears a radially protruding element which is designed to cooperate in use with a corresponding mating recess provided on a holder of a medical apparatus, the radially protruding element comprising side walls emerging from external surface of the end connector and a terminal wall extending transversally to said side walls.
 91. A rigid tubular piece according to claim 90, wherein said intermediate portion has a constant cross section and is at least partially transparent to electromagnetic and/or acoustic waves, said intermediate portion having a toric cross section.
 92. A rigid tubular piece according to claim 91, wherein the side walls of the protruding element are slightly converging when moving apart from the external surface of the end connector, and said terminal wall is substantially flat and parallel to a longitudinal axis of development of the rigid tubular piece.
 93. Cassette assembly comprising a rigid tubular piece according to claim 90, wherein the rigid tubular piece is in one piece with a rigid plastic casing having front and rear walls spaced from each other and a side wall between said front and rear walls to define a first fluid flow chamber, and wherein the rigid casing and the rigid tubular piece are in transparent plastic material, said rigid casing also comprising an intermediate wall extending between the side walls to define a second fluid flow chamber.
 94. Cassette assembly according to claim 93 wherein the rigid casing and the rigid tubular piece are in transparent plastic material.
 95. Cassette assembly according to claim 93, wherein said rigid casing also comprises an intermediate wall extending between the side walls to define a second fluid flow chamber. 